Method of heat treating magnesium base alloys



Patented Jan. 2, 1940 PATENT OFFICE METHOD OF HEAT TREATING MAGNESIUM I BASE ALLOYS Robert T. Wood,ieveland, Ohio, assignor, by

memo assignments, to

Ma esium Development Corporation, a corporation of Delaware I No Drawing.

This invention relates to magnesium base alloys and it is especially concerned with accelerating the heat treatment of certain cast alloys.

Magnesium base alloys which contain alu- 5 minum and zinc as the chief added alloying ingredients have been' used for some time in the production of castings because of their good casting characteristics and relatively high strength as compared to other magnesium base alloys. Although castings of these alloys are suitable for many purposes in the as-cast form yet it has been found that their strength can be materially increased by subjecting them to a solution heat treatment followed by rapid cooling to room temperature. It ordinarily requires a heating period lasting for a number of hours to secure the desired degree of solution of the soluble constituents in the cast alloys. In some instances, as for examplein the case of cast mago nesium base alloys of the type containing from 1 to 12 per cent aluminum, and 0.3 to 5 per cent zinc, it has been found'to be necessary to employ a two-step thermal treatment consisting of an initial heating at about 630 F. for several hours followed by heating at about 720 F. for a longer period of time, to obtain the desired degree of solution of the alloy ingredients. Ex

posure 'of the castings to higher temperatures for a shorter period of time as a means of reducing the time of heat treatment has been considered to be impractical because of the danger of intergranular melting and oxidation with consequent reduction in the mechanical properties of the casting. v

It is an object of my invention to provide a means of hastening the solution heat treatment of cast magnesium base alloys containing from 1 to 12 per cent aluminum and 0.3 to 5. per cent zinc. Another object is to avoid the necessity 40 for using the two-step thermal treatment which has heretofore been the standard practice in treating castings of this type." Still another object is to obtainsubstantially the same strength in solution heat treated cast magnesium base 48 alloys containing aluminum and zinc in a much shorter time than has been considered possible heretofore. a

My invention is based upon the discovery that the addition of from about 0.03 to 0.3 per cent 50 calcium to cast magnesium base alloys containing from about 1 to 12 per cent aluminum and 0.3 to 5 per cent zinc permits a more rapid solution heat treatment of the castings at a higher temperature than has hitherto been regarded as as being a safe practice. In the case of ca i g Application September 1, 1931, 'Serial No. 161,948

(o1. ins-21s) containing calcium it is necessary to use only one solution heat treatment instead of the customary two-step process. Furthermore, it has been found that neither the presence of calcium nor the shorter heat treatment reduces the strength 5 of the castings as compared to similar castings containing no calcium and treated by the two step method.

Although the addition of calcium is beneficial to alloys containing aluminum and admin the 10 amounts mentioned above, it is especially advantageous in alloys containing from 4 to 8 per cent aluminum and 2'to 4 per cent zinc; It is also my preferred practice to add from 0.03 to 0.1 per cent calcium to these alloys as well as those containing larger amounts of aluminum and zinc. Some alloy compositions that are particularly recommended for the production of heat treated castings contain 6 per cent aluminum, 3 per cent zinc and 0.05 per cent cal- 20 cium; 10 per cent aluminum, 1 per cent. zinc and. 0.05 per cent calcium.

The alloys may be made in the usual manner through the use of rich alloy hardener or by the addition of the pure metal. The alloys should 25 be melted and cast under the conditions usually prevailing in magnesium alloy foundries, that is, the melt should be protected against oxidation and the sand molds should be treated to prevent a burning of the metal. The alloys may be cast 30 within the customary range of pouring temperatures. y

The castings should be heat treated by sub: jecting them to a temperature of 720 to 780 F. for a period of 5 to 30 hours, followed by a rapid 35 cooling to below 400 F., generally to room temperature. If desired they may be artificially aged in the usual manner after the foregoing thermal treatment. In my preferred practice the castings are heated at 750 F. for 10 hours and 40 then rapidly cooled to room temperature. I have found-that it is necessary to heat alloys of the foregoing type to at least 720 F. to secure a solution of the soluble constituents within a reasonable length of time. At temperatures Q above 780 F., however, there is danger of inclpient melting and/or intergranular oxidation with a consequent weakening of the cast article. As to the length of time required for heating the castings, it hasbeen my experience that at least. 5 hours is needed to obtain a substantial degree of solution and that a complete solution is obtained within 30 hours.

Although the presence of 0.03 to 0.3 per cent calcium is useful in reducing the time required for heattreatment of the cast alloys by reason of the fact that higher temperatures can be employed, yet in greater concentrations than 0.3 per cent this element hinders or may even prevent the solution of the MgaAlz constituent in the ailoy. I have observed, for example, if more than 0.3 per cent calcium is present that the solubility of the intermetallic compound, MgaAlz, is so reduced that a satisfactory solution and in- ,crease in strength of the alloy can not be obtained within a reasonable time even at a heat treating temperature as high as 810 F. On the other hand, less than 0.03 per cent of this element has no substantial effect upon the solubility of the aforesaid compound, and it also is inefiective as far as decreasing the time required for heat treating the alloy is concerned. In the 7 range between 0.03 and 0.3 per cent, however,

there is an eflect upon the solubility of the MgaAl-z compound but it is subordinate to the advantage gained in reducing the time required for the solution heat treatment. In other words, the increase in the rate of solution gained by using higher heat treating temperature than has been heretofore employed in commercial practice more than offsets the solution impeding efiect oi 0.03 to 0.3 per cent calcium.

Having thus described my invention and certain embodiments thereof, I claim:

1. A method of producing, in a heat-treated magnesium base alloy containing 1 to 12% aluminum and 0.3 to 5% zinc, properties commensurate I aieaate num and 0.3 to 5% zinc, properties commensurate with those obtained by a two-step solution heat treatment, the method comprising adding 0.03 to 0.3% calcium to said alloy, casting the resulting alloy, subjecting the resulting casting to a single step solution heat treatment at a temperature between 720 and 780 F. for 5 to 30 hours, and rapidly cooling said heat-treated casting to a temperature below 400 F.

3. A method of producing in a heat-treated -magnesium base alloy containing from 4 to 8% aluminum and 2 to 4% zinc, properties commensurate with those obtained by a two-step solution heat treatment, the method comprising adding 0.03 to 0.1% calcium to said alloy, casting the resulting alloy, subjecting the resulting casting to a single step solution heat treatment at a temperature between 720 and 780 F. for 5 to 30 hours, and rapidly cooling said heat-treated casting to a temperature below 400 F.

4. A method of producing, in a casting of a magnesium base alloy containing 1 to 12% aluminum, 0.3 to 5% zinc, and 0.03 to 0.3% calcium, properties commensurate with those obtained by a two-step solution heat treatment, the method comprising subjecting the casting to a single step solution heat treatment at a. temperature between 720 and 780 F. for 5 to 30 hours, and rapidly cooling said heat-treated casting to a temperature below 400 F.

5. A method of producing, in a casting of a magnesium base alloy containing from 4 to 8% aluminum, 2 to i% zinc, and 0.03 to 0.1% calcium, properties commensurate with those obtained by a two-step solution heat treatment, the method comprising subjecting the casting to a single step solution heat treatment at a temperature between 720 and 780 F. for 5 to 30 hours, and rapidly cooling said heat treated casting to a temperature below 400 F.

ROBERT T. WOOD. 

